The most commonly used method for production by artificial lift is use of a pump jack--rod pumping system. In rod pumping, a pump jack is used to vertically reciprocate a rod extending down to the production zone of the well. The rod is connected to a subsurface pumping unit which consists of a piston in a pump barrel connected to the rod to reciprocate within the barrel and lift the fluid. The dependability and economy of these pump jack systems makes these systems highly desirable and generally used.
In the design of these systems, it is generally accepted that the capacity of the pumping system will exceed the maximum production of the oil well as the production rate declines. As a result, if the systems are operated at maximum capacity, the system will become what is known in the industry as "pumped-off," reducing the efficiency of this system due to a partially filled condition in the barrel. The partially filled pump barrel is caused by the pump removing liquid faster than the well produces. In addition, the pumped-off condition can result in damage to the rod string and pump.
As a consequence, control systems are currently available for detecting the pumped-off condition and for controlling the operation of the pump in response to detection of this condition. The history of the development of various of control systems is outlined in the 1977 Society of Petroleum Engineers of AIME Paper entitled "Successful Application of Pump-Off Controllers SPE No. 6853." As is pointed out therein, a development of generally applicable pump-off control methods was complicated by pumping abnormalities not associated with pump-off such as gas interference, harmonic pumping speeds, down-hole friction, equipment vibrations, corrosion, changes in the reservoir performance and the like. Historically, attempts to solve the varied problems of an efficient pump-off control has taken on many forms.
The initial efforts to control pump-off are basically an attempt by a pump operator to match the pumping speed to the production rate of the well or reservoir. However, in order to obtain maximum production from the well, it is generally necessary to maintain the lowest possible fluid level in the well, and therefore the lowest possible back pressure on the formation. In order to assure a low average fluid level, it is necessary to provide a pumping system with a capacity in excess of the productive capacity of the well. The excess pumping capacity required to maintain the low fluid level ensures that pump-off will occur unless the pump is controlled in some manner. The first effort made to deal with the pump-off problem was to manually start and stop operation of the pumping system. In this approach the lease operator would estimate the amount of pumping time required to obtain maximum production from the well and maintain the fluid level as low as possible. This approach required a pumper periodically to turn the well pump off and on to regulate the pumping operation. This method suffered from the disadvantage of being less than exact and labor intense.
The first attempts to automatically control the operation of the pumping system were to install timers which automatically stopped and started the operation of the pumping system. For example, the time clocks would automatically operate the pump for a period of time every hour. Again, these systems suffered from the disadvantage of being inexact in that the operator was required to estimate the amount of pump operation which would maximize production. The tendency, of course, in these time systems was to over pump the well to assure not missing any fluids, thereby causing the inherent production maintenance problems.
As a result of the inaccuracies inherent in a system which estimate fluid level, methods have been developed for analyzing the loading on the pump rod to determine when the pump-off condition occurs. Since rod loading is directly affected by the pump loading, a number of characteristics of the rod loading can be used to detect pump-off of a well. Various portions of the rod load versus position relationship of a well has been utilized to sense pump-off. One example of such a system is found in the U.S. Pat. No. 3,951,209 to Gibbs issued Apr. 20, 1976 entitled "Method For Determining The Pump-Off Of A Well." In this method, a dynamometer is used to monitor the total power input to the rod string to sense when power input decreases to determine when the well pump-off occurs. This system determines the power input to the well pump by integrating the rod load as a function of displacement. When the actual horse power input to the top of the rod string falls below a set minimum, a computer can be utilized to transmit a signal which stops the pumping unit for a period of time. This system is sometimes called an on-off pump-off control in that the system senses the pump-off condition and terminates the pumping operation for a period of time. However, in this type of system, the pumping rate has to be set to exceed the production rate of the well. The system operates by pumping until the pump-off condition is reached and shutting pumping operations down until fluid re-accumulates in the well. However, as was previously pointed out for maximizing production, the fluid level in the well needs to be maintained as possible without reaching a pump-off condition. And thus during that period of time, when the pump is not operated and fluid is flowing from the formation into the well, production will be lost because the fluid level is at too high a level. Even though the Gibbs patent suggests that a computer can be used to constantly monitor and adjust the shut-in periods to minimize the loss of production, the system does not provide a means for maintaining the most efficient fluid level for purposes of production.
Two later patents provide variations of the on-off system taught in the Gibbs patent. The first is U.S. Pat. No. 4,015,469 to Womack, issued Apr. 5, 1977 entitled "Pump-Off Monitor For Rod Pump Wells." In the Womack patent the same off-on method is used, however, the method of determining when pump-off has occurred is somewhat refined. In Womack. instead of integrating the power over the entire stroke, only the power input during a portion of the stroke is considered. In this patent, Womack suggests that a considerable difference in energy input between the pumped-off and normal pumping condition can be found in the last quarter of the upstroke and the first quarter of the downstroke. As Womack points out, the difference between the energy input for the pumping condition and the energy for the pumped-off condition is usually only five to fifteen percent of the total power input and that errors in the measurement of the load of the rod string or displacement of the rod string can produce an error in the final results which may prevent sensing of the pumped-off condition by measuring only a portion of the stroke. Womack attempts to overcome problems present in an on-off system which compares against a set point to determine pump-off.
The second variation of Gibbs is found in the U.S. Pat. to Patterson No. 4,034,808 issued July 12, 1977 entitled "Method For Pump-Off Detection." Patterson likewise uses an on-off system and utilizes rod performance during only a portion of the pump's cycle to sense the pumped-off condition. Patterson suggests using only the first quarter of the downstroke of the differences in energy between the pumped-off condition and the pumping condition are substantial. Patterson utilizes this portion of the pump stroke to determine whether or not the pumped-off condition is present to shut the system off.
These on-off systems suffer from the disadvantage of inhibiting well production during the shut-down period and also require that the system reach the inherently damaging pump-off condition before the pump operation is controlled.
One attempt has been made to dynamically control the fluid level in the well and maximize production. That system is described in the U.S. Patent to David Skinner. No. 4,145,161, issued in 1977 entitled "Speed Control." This system utilizes a variable speed controller and electric motor to continuously control the rate of removal of fluid from the well. The Skinner system measures the total electrical power supplied to the pump motor and regulates the pump motor speed based upon the fact that as fluid level decreases the total power increases. To implement the system, Skinner pumps the well down at a predetermined speed and monitors the total electrical power supplied as the well pumps down. When the well becomes pumped-off, the proportionality between the power and speed can be determined and set for a point before pump-off establishing a proportionally constant to be used in operating that particular well. This method leaves three major shortcomings when in actual use. The first is that it has been found that the so-called proportionality constant is not in fact a constant over the pumping rates and is rather a relationship whose proportion varies with speed. When Skinner assumes that the relationship is a constant, error is inescapable. Skinner recognizes this problem and suggests avoiding selecting a point too close to pump-off without informing a person of skill how to avoid being too close or even how to tell when one is too close. Second, Skinner controls directly proportional to fluid height above full barrel. Skinner is incapable of controlling in the more effective range of fluid height between pump-off and full barrel. Finally, Skinner's system is subject to errors induced by changes in system supply voltage.